Solar panels are expected by their makers to last at least twenty five years. One of many lifetime-limiting conditions to be dealt with to enable such a long lifetime is hot spots on the panel. Hot spots may limit lifetime by causing damage to the panel due to heat generated and/or longer term degradation of the panel cell material due to diffusion aging. Failure modes include melting solder joints, pin holes or open circuits in a cell, and damage to the panel case. Some causes of hot spots are manufacturing related, such as an assembly flaw, substandard materials, contamination of a solar cell, and the always-present manufacturing variations. Though a panel may have been manufactured with flaws, it may well be serviceable for an extended time, though less than expected. Other causes are beyond the control of the manufacturer or installer. For example, some cells in a panel may be exposed to more or less sunlight than other cells due to partial shade, dirt or bird droppings in a localized area, temperature variations across a panel, and non-uniform aging of the diffusion regions from cell to cell.
The destructive effects of hot-spot heating may be circumvented through the use of a bypass diode. A bypass diode is connected in parallel, but with opposite polarity, to a solar cell. Under normal operation, each solar cell will be forward biased and therefore the bypass diode will be reverse biased and will effectively be an open circuit. However, if a solar cell is reverse biased due to a mismatch in short-circuit current between several series connected cells, then the bypass diode conducts, thereby allowing the current from the good solar cells to flow in the external circuit rather than forward biasing each good cell. The maximum reverse bias across the poor cell is reduced by the bypass diode to about a single diode drop, thus limiting the current and preventing hot-spot heating.
A typical circuit model of a solar panel is shown in FIG. 16. For clarity of explanation, the example is simply two cells in series. Obviously a typical panel has many more cells in series to form a “string”, and some have multiple strings in parallel. In the model of FIG. 16, each solar cell is modeled as a current source in parallel with a reverse-biased diode. The example of FIG. 16 includes a cell 1602 in series with a cell 1604, with bypass diodes 1610, 1612 respectively. The current of the model arises from the photodiodes 1606, 1608 when exposed to adequate light. We consider four cases related to solar cells that are equal and unequal in power capacity, each case in open and short circuit configurations. In a short circuit condition and with matched cells the voltage across both the solar cells and the bypass diodes is zero; the bypass diodes have no effect. When open circuit (also with matched cells) the short current from each cell forward biases the cell. The bypass diodes are reverse biased, and again, have no effect on the circuit.
Assume now that cell 1604 is shaded, thus has less power providing capacity than that of cell 1602. For the short circuit condition, some current flows from cell 1602, forward biasing the cell 1602. The bypass diode 1610 is again reverse biased and has no effect. The voltage of the good cell 1602 forward biases the bypass diode 1612 of the weak cell 1604, causing it to conduct current. The shaded cell 1604 itself is reverse biased with approximately a diode drop of about −0.5 volts. For the fourth condition, that is a weak cell 1604 and an open circuit, the shaded cell 1604 has a reduced voltage. The bypass diodes 1610, 1612 are reverse biased and have no effect.
In practice, however, one bypass diode per solar cell is generally too expensive and instead bypass diodes are usually placed across groups of solar cells. The voltage across the shaded or low current solar cell is equal to the forward bias voltage of the other series cells which share the same bypass diode plus the voltage of the bypass diode. The voltage across the unshaded solar cells depends on the degree of shading on the low current cell. For example, if the cell is completely shaded, then the unshaded solar cells will be forward biased by their short circuit current and the voltage will be about 0.6V. If the poor cell is only partially shaded, the some of the current from the good cells can flow through the circuit, and the remainder is used to forward bias each solar cell junction, causing a lower forward bias voltage across each cell. The maximum power dissipation in the shaded cell is approximately equal to the generating capability of all cells in the group. The maximum group size per diode, without causing damage, is about 15 cells/bypass diode, for silicon cells. For a normal 36 cell module, therefore, 2 bypass diodes are used to ensure the module will not be vulnerable to “hot-spot” damage.
Consider now a typical solar panel configuration and response to partial shading. A set of 25 modules connected in series form a nominal Vmpp of 467.5 V at 11.23 A or 5,250 W. Assume each module is constructed of three strings of 38 cells (mpp @492 mV, 3.743 A) each and the top middle and bottom of each string are connected. Between the middle of top and middle to bottom are bypass diodes (Vf 410 mV). If one cell became shaded or soiled to the extent that it's current dropped by 374 mA or more (10%) then two candidate operating points would be found by an MPPT scan for the string:
Approximately 467.5V @10.853 A or 5,075 W or
Approximately 457.7V @11.230 A or 5,140 W
Since the portion of the module with the shaded cell only produce 10.853 A, its bypass diode is forced into conduction forcing the bypass diode's 410 mV and the 9.350 V of the 19 bypassed cells to be subtracted from that modules voltage (total loss of 9.760V from the string of modules). Within the bypassed 19 cells the sum of the voltage across the good 18 cells plus the voltage across the shaded cell must equal −410 mV (the voltage across the bypass diode) at the current of the shaded cell (because all 19 cells are in series).
The solution is approximately 8.856V across the 18 good cells and −9.266V across the shaded cell @3.369 A or 31.2 w of power dissipation in the shaded cell. Note that a similar situation exists with the other two sets of 19 cells because they too are forced to sum to the −410 mV of the bypass diode.
The bypass diode has the difference of module string current minus the bypassed sections. The module is producing 97.026 W for a loss of 54% and dissipating an additional 100 was heat. A string monitoring means, for example an ADC, would record a 10V drop in nominal Vmp for the string. A technician dispatched to investigate would find a module operating at 9V when he expected 18V, no change in power when he cast a shadow across half of the module and that some cells in the module were abnormally hot (all standard trouble shooting observations). The technician may conclude that the module is below the 80% limit and assert that it has failed. However at the factory, this module would flash test as only 3.4% below nominal at 18.7V and 10.853 A or 203 w, although it would show a current step of 374 mA (3.3%) at about 8.940V.
The result of the reversal of one or more cells varies for differing solar cell technologies. For cells of a mono-crystalline type, there may be no lasting damage but a loss of efficiency. For cells of a thin-film construction, reversal of a voltage on a given cell is immediately catastrophic. As is seen, then, bypass diodes are a necessary and effective method for diminishing hot spots caused by partial shading or other causes for a weak cell. However, looking to FIG. 17, we see that the strings 1702, 1704, 1706, 1708, 1710, 1712 have an interconnect of conductors of a certain size which we will call size “X”. If the bypass diodes 1712, 1722 conduct, they can carry as much as 3× the current of one of the strings, therefore the conductor for each bypass diode is normally sized as 3× that of a single string conductor. The size of the bypass diode interconnect 1730, 1732 then, adds significant area to the minimum area for constructing a solar panel.
What is needed is a means for avoiding hot spots without bypass diodes and their attendant area increase of a solar panel.